1. Field of the Invention
The present invention relates to printers for printing to various types of recording paper, such as roll paper and cut forms, and to a print control method therefor.
2. Description of the Related Art
When printing to roll paper, which is widely used for receipt printing, and when printing to cut forms supplied as single-sheet forms or multi-part carbon-copy forms for slip printing, the printing conditions vary according to the type of recording paper and such precise specifications required in the printer as the paper thickness and print density. A printer capable of handling roll paper is typically needed for receipt printing, and a printer equipped with a thermal print head is needed if the roll paper is heat sensitive paper. On the other hand, if cut forms are to be printed, then a printer capable of handling cut forms is needed, and if the cut form is a multi-part form, then a printer equipped with a wire dot print head is typically required. This has led to the development of or research into wire dot printers capable of supplying roll paper and cut forms, and printers equipped with a thermal print head for receipt printing and a wire dot head for slip printing, so that plural types of recording paper can be utilized.
In any case, the thermal print head or wire dot print head driving voltage is normally supplied from a common power source or supply such as an AC power source coupled to a switching regulator, a series regulator, or other type of DC stabilized control. Such stabilized power sources have internal impedance or resistance, and the output voltage changes with changes in the load applied to the output. In addition, because of the impedance characteristics wiring connecting the power source to the print head drive circuit, the driving voltage supplied to the print head drive circuit is often not fixed but fluctuates to some extent. Therefore, assuming a constant length of time for the application of the driving voltage to the print head, the energy delivered to the print head does not remain constant, resulting in a certain level of unevenness in the printing output quality which may be discerned by the user.
In order to make the energy imparted to the print head constant, a control method that varies the print head driving time such that the print head is driven for a longer period of time when the voltage is low, and the print head is driven for a shorter period of time when the voltage is high, is known. A graphical representation of the relationship between print head driving voltage and driving time is presented in FIG. 12.
From FIG. 12 it is readily apparent that the relationship between the driving period T1 to the driving voltage V1 has a roughly 1-to-1 correlation. By controlling the print head so that the print head driving time is more suited to the print head driving voltage, printing quality can be stabilized.
Unfortunately, this simple type of stabilization does not apply as well to printers that are capable of accommodating multiple types of print media or paper. That is, when the thickness and material of the recording paper being used varies, it is difficult to consistently obtain high print quality using the above method of matching driving voltage to driving time using a functional relationship. For example, the print head driving time can be relatively short with roll paper because it is a single sheet of paper, but when multiple layers of paper are used, as for multi-part forms and copies, the print head driving time must be increased to ensure proper recordations. When the printer has only one print head driving mode, however, the print head operates for the same print head driving time at a same given print head driving voltage. This means that if print head driving is set for multi-part forms, the print head driving time for roll paper becomes longer than necessary, causing such problems as increased printing noise and print head overheating. On the other hand, if print head driving is set for single sheet paper, the driving time is shorter than what is required for multi-part forms, and the ability to make copies is severely degraded.
In order to solve these problems, printers are being developed which control the length of time voltage or power is applied to the print head according to the relative thickness of the recording paper being used. Examples of such printers are found in Japanese Laid-Open Patent Publications 53-118315, 03-23953 and 03-93549.
The printer disclosed in 53-118315 is provided with a detector for sensing the presence of a paper thickness indicator affixed to the recording paper, and a magnet energizing time indicator signal which corresponds to the paper thickness. This signal is used to automatically determine the thickness of the recording paper and control the print magnet energizing current and print head driving time according to the sensed paper thickness.
The printer disclosed in 03-23953 has a special gap motor which is used to adjust the, gap between the printer platen and the print head, and a gap sensor to determine the current size of this gap. By measuring the difference in gap size between the point when no recording paper is present and when recording paper is present on the platen, a relative thickness for the recording paper is determined. From this determination of paper thickness, the length of time for driving the print head is selected.
The printer disclosed in 03-93549 also has a motor which is used to adjust the platen gap, along with a sensor for determining the maximum physical gap size for the printer. By measuring the distance by which the print head is moved from the maximum open gap configuration until it contacts the recording paper, such as by monitoring the rotation of the motor, the thickness of the recording paper can be detected. Once the thickness is measured, the length of time over which power is applied to the coils to drive print head pins is adjusted accordingly.
However, the printers disclosed in each of these publications require a special mechanism to detect the thickness of the recording paper. That is, in '315 publication, a paper thickness indicator affixed to the recording paper along with an associated detector are contemplated. For the printers disclosed in the '953 and '549 publications, both a platen gap adjustment motor and an associated gap sensor are required. As a result, it is easy to achieve high quality printing with particular types of recording paper because the print head driving voltage and print head driving time are automatically adjusted according to the paper thickness. It should be recognized, however, that these printers cannot accommodate differences in the printing, modes for single sheet paper and multi-part forms, or the printing modes for two color thermal paper that is heated one temperature to print black and another temperature to print red, for example. Further, conventional printers also cannot accommodate the different print head driving voltages and print head driving times required for multi-part forms and single sheet forms of the same thickness.
The aforementioned conventional printers are therefore unable to accommodate plural types of recording paper, and are even unable to generally accommodate the same type of recording paper because the precise characteristics of the paper may vary depending on recording material quality and manufacturing techniques employed, as is well known in the art. In fact, providing such specialized mechanisms conversely complicates the mechanical configuration of the printer, thus leading to degraded reliability and ease of assembly, and increasing production costs in a market known for strong price competition between products.